The ever increasing amount of mobile data traffic in mobile communication networks raises requirements on the infrastructure of the networks. In order to handle the traffic increase, radio cells are becoming smaller and smaller as a complement to existing macro networks (e.g. covering hotspots such as cafes and stores)
Radio access network (RAN) transport, i.e. communication between Node B and radio network controller (RNC), is becoming a bottleneck due to the high cost of providing connectivity with high quality transport. Under these conditions, operators seek to connect the Node B's using a wide variety of transport technologies (such as Asymmetric Digital Subscriber Line (ADSL)) with varying characteristics in terms of e.g. delay.
As a mobile terminal traverses between the cells, soft handovers occur on a per second basis in a city environment, i.e. communicates simultaneously with a plurality of Node B's having different transport characteristics. Under these circumstances, it is important to continuously aim for optimal performance and thus decrease roundtrip time in order to improve speech perception and throughput.
In order to provide data transmission with uniform rate over radio interface and to core network, data need to be buffered in the RAN nodes, e.g. in the RNC is case of uplink communication and in the Node B's in case of downlink communication. In case of communication links with large delay difference, data have to be buffered for the link with the shorter delay to take into account the delay of the communication link having the greatest delay.
Thus, reception window size and position must to be selected with respect to expected data frame delays and delay variations on the micro-diversity legs.
In case of transport with varying delay characteristics, the size of the reception window must be large enough with appropriate margins in order to avoid frame drops.
As a consequence of a large reception window, timing adjustment procedure for adapting to various delay conditions will be less sensitive. There will be more frames arriving within the reception window and as a result fewer timing adjustment control frames sent from the Node B's to the RNC, wherein transmission time adjustment of frames sent from the RNC to the Node B's is undertaken more seldom. Hence, data will be buffered for longer periods, which will increase the RTT. It should be noted that data frames are sent from the RNC such that they reach the reception window of the NodeB at an end point of the window in order to reduce buffering of data frames. Thus, when link delay increases, frames will arrive outside the reception window and timing adjustments are undertaken accordingly. However, if the delay decreases, data frames will arrive in the window but not at the end point. In case of a great decrease, the data frames will possibly arrive in the middle of the reception window. In this case, no timing adjustments will be undertaken, resulting in unnecessarily large delays in the communication link.
In the art, round trip delay between an RNC and Node B's participating in handover a mobile terminal is measured once and for all initially, and subsequently variations in delay on the communication links established between the RNC and the Node B's is adjusted either by controlling timing of the data transmissions from the RNC to the Node B's or, if the delay exceeds a Transmission Time Interval (TTI), by adjusting connection frame number (CFN) associated with radio frames sent between the RNC and the Node B's, or both.